[SPARK-23040][CORE][FOLLOW-UP] Avoid double wrap result Iterator.

## What changes were proposed in this pull request?

Address apache#20449 (comment), If `resultIter` is already a `InterruptibleIterator`, don't double wrap it.

## How was this patch tested?
Existing tests.

Author: Xingbo Jiang <[email protected]>

Closes apache#20920 from jiangxb1987/SPARK-23040.

Author: jiangxb1987

core/src/main/scala/org/apache/spark/shuffle/BlockStoreShuffleReader.scala

@@ -111,8 +111,13 @@ private[spark] class BlockStoreShuffleReader[K, C](
       case None =>
         aggregatedIter
     }
-    // Use another interruptible iterator here to support task cancellation as aggregator or(and)
-    // sorter may have consumed previous interruptible iterator.
-    new InterruptibleIterator[Product2[K, C]](context, resultIter)
+
+    resultIter match {
+      case _: InterruptibleIterator[Product2[K, C]] => resultIter
+      case _ =>
+        // Use another interruptible iterator here to support task cancellation as aggregator
+        // or(and) sorter may have consumed previous interruptible iterator.
+        new InterruptibleIterator[Product2[K, C]](context, resultIter)
+    }
   }
 }